1. Field of the Invention
The present invention relates to a photoelectric conversion device, an imaging system, and a method for driving a photoelectric conversion device.
2. Description of the Related Art
Japanese Patent Application Laid-Open No. 2004-134867 discloses a plurality of pixel units each including an amplification MOS transistor provided in common for two photodiodes. Japanese Patent Application Laid-Open No, 2004-134867 further discloses that the two photodiodes included in the pixel unit are connected to the common amplification MOS transistor.
In Japanese Patent Application Laid-Open No. 2004-134867, electric charge is generated by one of the two photodiodes included in the pixel unit, and the electric charge is transferred to a gate node of the amplification MOS transistor to read a signal from the pixel unit. Electric charge generated by the other of the two photodiodes is transferred to the gate node of the amplification MOS transistor, to read a signal from the pixel unit, the signal corresponding to the total amount of electric charge generated by the two photodiodes.
A technique for providing an analog-to-digital (AD) converter corresponding to each of columns of a pixel array in which pixels are arranged in a matrix form is known. As an example, it is known that the AD converter provides a digital signal by comparing a reference signal monotonously changed with time and an analog signal with each other.
If an AD converter is provided at each of vertical signal lines, in an imaging device described in Japanese Patent Application Laid-Open No. 2004-134867, time required for AD conversion of a signal from two photodiodes is twice the time required for AD conversion of the signal from one photodiode. Therefore, it is difficult to speed up the operation of a sensor by simply providing the AD conversion circuit for each column of the pixel array.